The present invention relates generally to button-size electrochemical cells, and in particular relates to button-size metal-air cells of the zinc-air type having an increased internal volume for active material.
Metal-air cells are used for a variety of applications. A large fraction of such cells are used in hearing aids. Newer versions of such hearing aids are placed inside the outer portion of the human ear, whereby any leakage of material from the cell may come into contact with the skin of the wearer, in the wearer's ear. Accordingly, it is critical that such cells do not leak any of the alkaline electrolyte which is routinely used in such cells.
Furthermore, to the extent that such leakage is detected in the manufacturing process, quality control efforts must be expended to reduce or preferably eliminate such leakage wherever practical. Such quality control efforts, of course, bear associated costs. Therefore, any advance in reducing the leakage of electrolyte is significant to the health of the consumer of such cells, as well as to the commercial value of such cells to the manufacturer.
Furthermore, the overall volume of the cell, useful for containing electrochemically reactive materials, is limited to that space not occupied by non-reactive components of the cell. Thus, to the extent use of internal cell volume by non-reactive components can be controlled or reduced, additional electrochemically reactive material may be used in the cell, and the use life of the cell thereby extended. Accordingly, any effort expended in controlling leakage must be accompanied by a sensitivity to any reduction in the space which is available for use in containing electrochemically reactive materials in the anode can.
One potential source of leakage is the interface between the side walls of the anode can and the cathode can. A seal is therefore traditionally used that is in contact with both side walls and associated sealing fluids, or other materials, in order to successfully provide the necessary seal function. Such frictional engagement with the anode can is routinely obtained using a sliding assembly. However, in such conventional assemblies, the (plastic) seal may be damaged by the anode can. Such damage is difficult to detect, and thus such damaged cells may well leave the manufacturing facility undetected.
One cell overcoming the above-mentioned leakage, while providing controlled doming of the cathode assembly is described in U.S. Pat. No. 5,945,230, the disclosure of which is hereby incorporated by reference as if set forth in its entirety. In particular, referring to FIG. 1, a button-size metal air cell 10 includes an anode can 11 presenting an annular side wall 12 surrounded by a cathode can 13 to define an enclosure 14 filled with an electrolyte and active anode material. A radially compressed gasket 15 (i.e., where the gasket is compressed to a reduced thickness with respect to its free state by forces acting in the radial direction) provides a seal that electrically isolates the anode material from the cathode can 13. The gasket 15 extends substantially along the side wall 12 of the anode can 11 to prevent leakage of electrolyte. A cathode assembly 16 is disposed between the base of gasket 15 and cathode can 13, and a separator 17 layer 61 is disposed at the interface between the anode cathode assembly 16, and permits electrolyte transfer between the anode and cathode while providing electrical isolation therebetween. One or more of air ports 18 extends through the bottom of cathode can 13, thereby providing avenues for the transport of oxygen into the cell adjacent the cathode assembly 16. While the cell thus described is suitable for its intended purpose, the gasket occupies valuable space within the cell that could otherwise be occupied by active material to increase the life of the cell.
What is therefore needed is a cell having a seal that prevents electrolyte leakage while increasing the volume of active material than presently achieved.